This invention relates to methods of metallizing beryllia substrates to enable soldered connections to be made to the metallized film. In particular, this invention relates to an improved method of metallizing beryllia substrates through direct autocatalytic deposition of a metallized film thereon which provides a strong mechanical bond between the substrate and film.
In the manufacture and use of semiconductors it is occasionally necessary or desirable to mount a semiconductor on a dielectric for electrical isolation of the semiconductor chip. Ceramics are preferable dielectrics for this purpose because they also possess good thermoconductivity properties useful in removing heat from the semiconductor when in operation. Beryllia (beryllium oxide) has the highest thermoconductivity of all ceramics and is therefore preferred in applications where good heat dissipation is a significant criteria. However, beryllia is also one of the most expensive ceramics.
Semiconductors are conventionally mounted to the ceramic substrates by soldering, thereby requiring that the substrate by provided with a metallized film for obtaining a solderable surface. Presently known methods for metallizing beryllia exist in the vacuum technologies such as plasma spraying, ion plating and sputtering. A more conventional method involves firing another ceramic, moly-manganese, to the beryllia as an intermediate substrate which is then platable by autocatalytic deposition plating processes to achieve the metallized film surface. These methods each involve capital and operational expense which adds to the high cost of the beryllia substrate.
The economic burdens associated with the use of beryllia as a semiconductor mounting substrate can be significantly reduced by directly plating the substrate by conventional autocatalytic deposition methods. Attempts to accomplish this have, however, provided unsatisfactory bonds between the substrate and the metallized film which in turn provide an inadequate mechanical bond of the semiconductor to the substrate. The invention disclosed herein primarily resides in the discovery of the particular properties of a beryllia substrate which contribute to the aforementioned unsatisfactory bonds and in recognizing a manner in which to operate upon these properties to render the beryllia substrate fully suitable for metallization by direct autocatalytic deposition.
Beryllia substrates are available in various grades determined by the contents of the beryllia in proportion to other materials present in the substrate. The thermoconductivity properties of the substrate decrease as the content of other materials increases. Thus, for applications of beryllia substrate as a semiconductor mount having good thermoconductivity, it is preferable to use a wafer type substrate wherein the beryllia content is very high. This material is manufactured in at least one commercially available embodiment by forming a slurry of beryllia with silica and magnesium, spreading this slurry in the desired thickness on a film sheet such as mylar, allowing this slurry to harden, fusing the material under high temperatures (sintering), and then cutting it to the desired size. The surface of the substrate that was in contact with the mylar film during the curing process is very smooth, and the surface that had been exposed is only slightly rougher.
In attempting to metallize a wafer of the aforedescribed beryllia, it was noted that greater adhesion occurred on the rougher surface, although the bond was unsatisfactory for enabling a reliable soldered connection to be made to the metallized film. Inasmuch as beryllia is an oxide and as such will not establish a surface-to-surface bond with a metal by a chemical process, any adhesion of the metal to the substrate is apparently due to a roughened surface profile of the substrate.